In Data: Breathing in Delhi is like smoking how many cigarettes?

NEW DELHI - Pollution readings in India's capital New Delhi hit their highest in this pre-winter season, forcing the government to restrict vehicle movement and construction, and urging schools to give classes online.

India's Central Pollution Control Board showed Delhi's 24-hour air quality index (AQI) reading hit 494 on a scale of 500 on Monday.

Just breathing with that amount of smog is equivalent to smoking 27 cigarettes, according to an online calculator based on a formula developed by U.S. non-profit Berkeley Earth.

"Air pollution is arguably the greatest environmental catastrophe in the world today," Richard A. Muller and Elizabeth A. Muller said in their 2015 study 'Air Pollution and Cigarette Equivalence'.

At least five stations in the capital reported an AQI of 500 the same day, lowering visibility to 100 m (328 ft) and diverting many flights.

India battles air pollution every winter as cold, heavy air traps dust, emissions and smoke from farm fires started illegally in the farming states of Punjab and Haryana.

Swiss group IQAir ranks New Delhi as the world's most polluted city and said its air quality was "hazardous".

The concentration of PM2.5 - particulate matter measuring 2.5 microns or less in diameter that can be carried into lungs, causing deadly diseases and cardiac issues - was nearly 131 times the World Health Organization's recommended levels, it said.

About 7.2% of all deaths in India are attributable to a daily exposure to PM2.5, according to a July study by The Lancet.

This equals about 92 deaths a day in India.

(Reporting by Annie Banerji; Editing by Jon Hemming.)